Noisy motor after adding decoder

[Barrowman]

I have recently fitted a sapphire decoder to a Hornby DCC ready railcar. It functions okay apart it suffers from a very sick sounding "gritty" noise when running at low speeds or in building-up speed. It is not present (or maybe, not so noticeable) at

higher speed and it did not make the noise prior to chipping.

I raised the issue at my local club and a colleague mentioned that he'd also had this problem with his railcar in the past... albeit fitted with a Bachmann decoder. He had managed to resolve

the problem by changing a CV - the number of which escapes me at the moment - maybe 154 or 158? but certainly higher than those listed in the Hornby Sapphire handbook.

We did try swapping the Hornby Sapphire decoder for a TCS (I think!) decoder and

the noise had miraculously vanished.

Has anyone else come accross this problem or, more importantly!, have any suggestions as to how it might be resolved.